A large number of different types of radiation sensing elements have been developed. One such device is a Geiger-Mueller detector. Basically, it consists of a pair of electrodes surrounded by a counting gas especially selected for the ease with which it can be ionized. When radiation ionizes the gas, the ions so produced travel toward the electrodes between which is maintained a high electrical potential. The motion of the ions toward the electrodes constitutes an electric signal which can be detected and recorded electronically. Thus, each particle or ray of radiation passing through the Geiger-Mueller tube which ionizes the counting gas produces an electrical signal, the number of such signals being a measure of the intensity of the radiation.
Geiger-Mueller detectors are available in a variety of forms such as the "side-window" or "end-window" type of tube which are so named because they have a thin window at either one side or at one end through which the radiation passes. The end-window type consists of a metal cylindrical envelope or one made of glass the inside of which has been coated with a conducting material. The wall of the tube constitutes the negative electrode known as the cathode. In the center, concentrically aligned, is a fine metal wire which serves as the anode.
The space between the electrodes is filled with a counting gas, such as helium or argon which can be used along with a small amount of a polyatomic gas such as alcohol or butane, if internal quenching is desired. However, the polyatomic gas is not needed if the detector is quenched externally. The window prevents the escape of the gas to the atmosphere, yet is sufficiently thin so that it allows the passage of certain types and energy of ionizing radiation into the tube. This type of tube is most useful for detecting moderate to high energy beta particles.
Other types of radiation sensing elements include the proportional detector, the ionization chamber detector, and a scintillation detector. Each of these detectors differs in its mode of operation and in its sensitivity to a particular type of radiation. They are similar in that they convert ionizing radiation into electrical signals.
A scintillation detector is used in a liquid scintillation counter to detect the radiation emitted from a sample (potentially contaminated with radioactive material) which is introduced into the counter. In this system a contaminated sample is placed in a vial containing a mixture consisting of scintillation fluor and a solvent. The vial is then introduced into a dark chamber where emitted photons caused by the interaction of ionizing radiation and the fluor are detected and counted. There are a number of disadvantages associated with this method: the instrument is expensive; it is so large that it is not portable and the samples to be evaluated must be brought to it, for fixed contamination this would require defacing an object to obtain a sample; the instrument is sensitive (requiring it to be located remote from areas where radioactivity is handled) and complex, needing regular maintenance; there is a delay between when the sample is prepared and counting is effected; it is designed to count batches of samples and is inefficient to use for evaluating individual samples; and it is necessary to purchase, store and dispose of chemicals which can have additional disadvantages in being expensive, flammable, toxic, and also necessitate the disposal of hazardous waste.
Liquid scintillation counters are useful to detect low energy radiation which cannot enter a closed window Geiger-Mueller type tube. However, open window Geiger-Mueller tubes are capable of detecting low energy radiation because radiation can enter the tube. Counting gas is continually supplied to the ionization chamber to replenish the gas which escapes through the open window. Such detectors function by placing a sample close to the open window. This is needed because low energy radiation cannot penetrate across a wide air gap. For example, beta radiation produced by tritium can pass only through about one-third of an inch of air at atmospheric pressure.
The disadvantages associated with open window Geiger-Mueller tubes include the following: it is necessary to place the sample next to an open window of the chamber containing an exposed electrode having a potential of about 900 to 1200 volts, thus creating an electric shock hazard; high gas flow and/or constricting the size of the open window is necessary to provide a complete envelope of counting gas around the electrode; high gas consumption is expensive and can require an expensive gas supply manifold entailing use of multiple tanks of gas or frequent interruptions to replace empty gas tanks; instrument start-up requires purging the chamber to displace accumulated air which causes a significant delay before a sample can be counted (premature counting could give a false negative result, i.e., that the sample is free of contamination when in fact it is contaminated); the instrument is difficult to use because any movement of the detector or of air near the detector can displace the counting gas from the electrode region thereby interrupting detection capability. This is not practical when personnel contamination is involved and is unlikely to be practical for sampling objects and facility surfaces.
Open window gas flow proportional counting is similar to the Geiger-Mueller method described above. One important difference is that the proportional detector employs a different electrical potential in order to distinguish among the various types of radiation and, thus the efficiency of the detector is significantly diminished. An example of such a counter is the windowless tritium surface contamination monitor, models PTS-65 and PTS-6M, sold by Technical Associates, 7051 Eton Avenue, Canoga Park, Calif. 91303. Disadvantages associated with this method include the following: the need for sophisticated and expensive electronics which are usually not interchangeable with other commonly used detectors; requires extensive training in order to operate properly; more complex calibration techniques are involved.
Today, it is typical to use liquid scintillation counters for detecting samples which can be easily removed from a surface. Portable thin window proportional counters are used to monitor surfaces suspected to be contaminated with alpha emitters, Non-portable proportional counters are used with samples which can be easily removed from a surface. Portable thin window Geiger-Mueller detectors are used to monitor surfaces suspected to be contaminated with at least one radionuclide emitting moderate and/or high energy beta, gamma, and/or X-radiation.
U.S. Pat. No. 4,633,089, issued to Wijangco et al. on Dec. 30, 1986, describes a hand held radiation detector for measuring localized radiation at low levels of the order of one count per minute which utilizes a sealed chamber defined by a housing.
U.S. Pat. No. 4,644,167, issued to Sorber on Feb. 17, 1987, describes a radiation dose rate measuring device.
U.S. Pat. No. 4,409,485, issued to Morris et al. on Oct. 11, 1983, describes a radiation detector and method of opaquing the mica window.